Electrical resistor



y 1942 H. J. LOFTIS ET AL 2,289,791

ELECTRICAL RES ISTOR Filed ,July 20, 1940 INVENTORS I ,7 ATTORNEY.

Patented July 14, 1942 UNlTED STATES ATEN T QFFICE 'ELECTRICAL'RESISTORHomer J. 'Loftis and John M. Apple, Ironton,

Ohio, assignors to Henrite Products Corporation, Ironton, Ohio, acorporation of Ohio Application July 20, 1940, Serial No. 346,564

4 Claims.

confined locations or restricted space, and whereby the same are adaptedto carry comparatively much higher loads than heretofore possible withprior forms of resistors.

Carbon resistors of various values of resistance are usually produced bymixing an inert nonconductive material (e. g. finely ground mica) and acarbonaceous material (e. g. carbon black) in'selected relativeproportions together with a suitable bonding agent (e. g. phenolicresin), and thereupon molding the mix into desired shape and size. Theorder of magnitude of resistance .for agiven size of resistor is usuallycontrolled by varying the relativeproportions of the inert andcarbonaceous ingredients of the molding compound or mix, the percentageof bonding agent being ordinarily held approximately the same.

It has heretofore been standard practice in the art to use carbonresistors on comparatively light loads or when the number of watts to bedissipatedby'a resistor unit is small. Examples of service involvingsuch conditions are for use in radio equipment where the wattage isusually one watt or less per resistor, or in connection with automotivestarting and lighting generator .voltage regulators where the wattageper resistor is ordinarily approximately five watts. The reason forlimiting the use of carbon resistors to conditions involving but lowwattage is that the maximum temperature of the resistor must be keptdown so as to prevent possibility of excessive carbonization of thebonding agent, which, if permitted to occur, would result in decreasingthe resistance of the unit, thus rapidly altering the original orspecific resistance desired to be maintained, and ultimately causing theresistor to fail or even burn up because of the 'high temperatureattained.

The conventional carbon resistor has heretofore been made in elongatedbody form of relatively small cross-sectional area compared with itslength. Such body forms are usually mounted between end supportingbrackets or spacers, or carried by wire leads. Byreason of such bodyforms and methods of support, the major part of the heat generated inthe resistor could only be dissipated by radiation, since but a minorpart of said heat could escape by conduction through the small supportsor wire leads, and since dissipation of the heat by radiation is slowand inefficient, the trapped heat causes the resistor temperature torapidly mount to a high or excessive range, with resultant rapiddiminution of specific resistance desired to be maintained, as well aswith risk of destroying the resistor due to burning thereof, all ofwhich is highly objectionable.

With a view of avoiding the objections above referred to, this inventionhas for an object to provide a novel form and construction of carbonresistor, the body of which is of disc-like shape, having a largediameter and cross-sectional area compared with its length orthickness;such disclike body'providing largeface areas adapted to be mountedrespectively in good contact with its supporting base and top contactplate or the like, whereby a greater percentage of heat generated in theresistor body will be rapidly carried away by conduction through thesupporting base and top contact plate or the like, thus maintaining asafe working temperature for the material from which the resistor bodyis made, and thereby eliminating risk of alteration of the specificresistance'desired to be maintained, as well as risk of destruction byburning.

Another object of the invention is to provide a disc-like resistor bodyhaving its face areas, and portions of its peripheral areasthere-adjoining, coated with a low resistance and relatively high heatconductive material, such e. g. as metallic material, thus asuring bothgood electrical and heat transfer contact thereof with the supportingbase and top contact plate or the like by which the same is operativelymounted and connected in the circuit to be served thereby.

Another object of this invention is to provide a disc-like resistor bodywhich can be easilyand accurately rated to a desired specific resistanceby cutting away the low electrically conductive material thereof at thecenter of its periphery by an amount calculated to accurately attain theexact total resistance required within the range ofthe particularcompound of which it is made; the depth and width of the annular outbeing so selectively determined as to desirably adjust the resistance ofthe unit within limits, as well as to compensate for small variationswhich may arise during the making up and processing of the compound ofwhich the resistor body is composed.

Other objects of this invention, not at this time more particularlyenumerated, will be understood-from the following detailed descriptionof the same.

An illustrative embodiment of the instant invention is shown in theaccompanying drawing, in which:

Fig. l is a side elevation of a disc-like carbon resistor according tothe invention, the same being shown as operatively mounted between asupport and top contact plate or the like; Fig. 2 is a top plan view ofthe same; Fig. 3 is a vertical sectional view of the same; and Fig. 4 isa plan view of the resistor body per se.

Similar characters of reference are employed to indicate correspondingparts.

In the drawing, the reference character III indicates the body of theresistor, the same being of substantially disc-like form of any suitableperipheral shape. By disc-like form is meant that the body is sodimensioned that it possesses a large diameter and cross-sectional areacompared With its length or thickness. The ratio of area to length (A/L)should be greater than unity.

The body I is made of a suitable compound of inert non-conductivematerial, carbonaceous conductive material, and a bonding agent; saidcompound being molded into a solid mass, either directly into therequired disc-like form, or to provide material from which saiddisc-like form may be cut and machined. For convenience in mounting theresistor body II], the same is provided with a central vertical throughopening or bore II. The top, bottom and external peripheral surfaces ofthe body III are provided with a low resistance but high heat conductivecoating I2 of metallic material. This coating I2 may be deposited uponthe body II] by electro-plating procedure, or by spraying the metallicmaterial thereon. The coating I2 is cut away from the periphery of thebody I0 intermediate the top and bottom faces of the latter, so as to beelectrically interrupted by a gap which is bridged only by theintermediate high resistance material of which the body is made.

To rate or adjust the resistance of the body within limits, the materialat the periphery thereof which is exposed by the gap between the coatingsections, is cut or trimmed away to remove an amount thereof calculatedto give the remainder the exact total resistance required within therange of the compound of which the body I0 is composed. Both the widthand depth of the peripheral regulating groove or channel I3 thus formedwill be factors in determining the adjustment of the resistance of theunit within limits. This will be apparent from the following formula:

R equals PL/ A wherein R denotes resistance of the unit;

P denotes specific resistance of the compound of which the unit iscomposed;

L denotes effective length of the unit (this being determined by thewidth of the regulating groove or channel I3) and A denotes crosssectional area of the unit (this being determined by the depth of theregulating groove or channel I3).

Illustrative of a convenient manner and means of mounting the resistor,the body II] is shown with its coated bottom face seated upon and. inelectrical contact with the base I4 or other suitable conductive part ofthe device to be served by the resistance, and to which a lead wire I5may be secured, if desired; said base I4 or the like being tapped toreceive a fastening screw I6. superposed upon and in electrical contactwith the coated top face of the body I0 is a perforate conductivecontact plate II, which, if desired, may be provided with an ear or lugI8 to receive connection thereto of a lead wire I9, Overlying saidcontact plate I! is an insulating washer 20. Inserted through saidinsulating washer 20 and contact plate I! to extend into and line thebore II of the body III is an insulating bushing 2|. The fastening screwI6, equipped with a lockwasher 22 beneath its head I6, is passeddownwardly through the bushing 2| and screwed into the base I4, thussecuring the body in service position and operative relation to thedevice to be served thereby. The bushing 2| and insulating washer 20serve to efficiently insulate said fastening screw and its lock-washerfrom the resistor unit.

It will be obvious that, by reason of the relatively considerable areaof the coated bottom and top faces of the body II] which are in contactwith the base I4 and with the contact plate I1, and by reason of theefficient heat conductive character of the sections of coating materialI2, the major percentage of heat generated within the resistor body inuse will be quickly transferred from the body I0 to the base I4 andcontact plate I1, and will be carried away therefrom by conductionthrough the latter parts, thus maintaining a safe working temperaturefor the material from which the resistor body is made. A carbonresistor, constructed and mounted in the manner above described, willprovide a very stable resistor unit when working at loads heretoforeconsidered impractical and generally impossible for the same volume orsize of material in body forms heretofore used in the art.

Having described our invention, we claim:

1. A method of regulating the effective resistance of a disc-likeresistor body having a coating of low electrical resistance materialcovering its top, bottom and peripheral surfaces comprising the steps.of cutting away portions of said coating contiguous to the periphery ofsaid body to interrupt said coating by an annular gap and then cuttingaway exposed peripheral portions of said body to form a diameterreducing circumferential channel therein, the width of said gap and thedepth of said channel being selectively predetermined to adjustresistance of the body within limits.

2. A resistor comprising, a body of disc-like form having highelectrical resistance characteristics, said body having a diameter andcross-sectional area substantially in excess of its length or thickness,a coating of low electrical resistance but high heat conductivecharacteristics adhered to and covering the top, bottom and peripheralsurfaces of said body, an intermediate annular portion of said coatingwhich covers the periphery of said body being cut away to provide a gapof selected width, whereby to adjust the resistance of the body withinlimits, while at the same time providing, by the remaining parts of saidannular portion together with the respective coating areas on therespective top and bottom faces of said body extending therefrom, meansto rapidly conduct away and dissipate heat from said body.

3. A resistor comprising, a body of disc-like form having highelectrical resistance characteristics, said body having a diameter andcross-sectional area substantially in excess of its length or thickness,a coating of low electrical resistance but high heat conductivecharacteristics adhered to and covering the top, bottom and peripheralsurfaces of said body, an intermediate annular portion of said coatingwhich covers the periphery of said body being cut away to provide a gapof selected width, as one factor adapted to adjust the resistance of thebody within limits, and peripheral portions of said body exposed by saidgap being cut away to provide a channel of selected depth as anotherfactor adapted to adjust the resistance of the body within limits, theremaining parts of said coating providing means to rapidly conduct awayand dissipate heat from said body.

4. A resistor comprising, a body of disc-like form having highelectrical resistance characteristics, said body having a diameter andcross-sectional area substantially in excess of its length or thickness,a coating of low electrical resistance but high heat conductivecharacteristics adhered to and covering the top, bottom and peripheralsurfaces of said body, an intermediate annular portion of said coatingwhich covers the periphery of said body being cut away to provide a gapof selected width, as one factor adapted to adjust the resistance of thebody within limits, and peripheral portions of said body exposed by saidgap being cut away to provide a channel of selected depth as anotherfactor adapted to adjust the resistance of the body within limits, theremaining parts of said coating providing means to rapidly conduct awayand dissipate heat from said body, a conductive base to contact thecoated bottom face of said body, a contact plate to engage the coatedtop face of said body, fastening means to bind said contact plate, bodyand base together, and means to insulate said fastening means from saidbody and contact plate.

HOMER J. LOFTIS. JOHN M. APPLE.

